(1) Field of the Invention
The present invention relates to a method and assembly for preferentially reducing the carbon monoxide content of a hydrogen rich stream consumed in a fuel cell power system.
(2) Description of the Prior Art
Fuel cell power systems have been used to supply power when internal combustions are not practical, such as in manned space vehicles. Fuel cell power systems have also been proposed as electrical vehicular power plants to replace internal combustion engines. Typically, a hydrogen stream is used for the fuel source for an electrochemical fuel cell that converts hydrogen at the anode or fuel electrode (using oxygen or air as the oxidant in the cathode process) to electrical energy.
The hydrogen used in the fuel cell can be derived from the reformation of methanol according to the reaction: EQU CH.sub.3 OH+H.sub.2 O.fwdarw.CO.sub.2 +3H.sub.2
In the methanol reformation process, methanol and water (steam) are reacted to generate hydrogen and carbon dioxide. This reaction is accomplished heterogeneously within a chemical reactor that provides the necessary thermal energy throughout the catalyst mass. One such forced convective reformer is disclosed in U.S. Pat. No. 4,650,727 to Vanderborgh.
A small amount of carbon monoxide is typically generated during the methanol reformation process. Carbon monoxide, even at low concentrations, must be removed from the hydrogen rich gas stream because carbon monoxide causes degradation within the fuel cell. Carbon monoxide is usually formed in concentrations between 1 and 3 mole percent, depending on process conditions.
It has been known that the carbon monoxide level of the gases emanating from a methanol reformer can be reduced by utilizing a "shift" reaction. In the shift reaction, water is added to the methanol reformer output gas to lower its temperature from about 250.degree. C. to about 190.degree. C. and increase the steam to carbon ratio therein. This increased steam to carbon ratio serves to lower the carbon monoxide content of the product gas by the shift reaction (This reaction occurs in the presence of a shift catalyst): EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2
Depending upon the flow rates and the steam injection rate, the carbon monoxide content of the gas exiting the shift reactor can be as low as 0.5%. Concurrently, almost all the residual methanol is thus converted to carbon dioxide and hydrogen, because of reaction on active sites in the shift catalyst.
Due to the poisoning of the fuel cell anode, even small concentrations of carbon monoxide (50 parts per million, and lower) can cause severe performance degradation in the fuel cell. This is illustrated by a decreased voltage output in the presence of carbon monoxide. Therefore, it is necessary to further remove carbon monoxide from the hydrogen rich stream prior to injection into the fuel cell device.